It has been said that the mountain tunnel has a good resistance to an earthquake, and we have not been very concerned with the behavior of the tunnel under earthquakes. But the result of case studies on earthquake damages of mountain tunnels shows that some tunnels were damaged by an earthquake. In fact, in the 1995 Hyogoken-Nanbu Earthquake, of 100 mountain tunnels in service within the disaster area, around 20 mountain tunnels suffered some influence of the earthquake and around 10 of them received some remarkable damage needing repair and reinforcement.
This paper focuses on the behavior of the tunnel portal during the earthquake and the damaging mechanism of it. Then, an effective way of reinforcing the tunnel is discussed on the basis of analytical results.